COIL BY-PASS FACTOR AND AIR MIXTURES
Cooling and dehumidification process involves not only sensible cooling but also latent removal or reduction of moisture content or dehumidification. Therefore the air will have to be cooled below its dew point temperature.
COIL BY-PASS FACTOR
In the proceeding section it is shown that the DB temperature of the air passing over a cooling coil tends to approach the surface temperature of the coil. If all the air passing through the coil come into intimate contact with the cooling surface and remains in contact with it for a sufficient length of time, the DB temperature of the leaving air could be decreased to the temperature of the cooling surface. However, as a practical matter, a certain portion of the total air quantity passing through any heating or cooling coil never comes into contact with the coil surface and therefore is unaffected by its passage through the coil. That is, it leaves the coil in the same condition that it enters the coil.
From the forgoing, it can be assumed that the air leaving the coil is actually a mixture of two air-streams or components . One component is the portion of the air that comes into direct contact with the coil surface and is assumed to leave the coil at a DB temperature equal to the mean surface temperature of the coil (line AB). The other component is the by-pass air, which does not contact the coil surface and it is assumed to leave the coil at the same DB temperature as when entering (line BC). The line AC then, represents the total quantity of the air mixture leaving the coil, which is the sum of the two component air quantities.
Since, in terms of the DB temperature scale, the length of line BC is equal to Tb
- Tc, and the length of line AC is equal to Ta
- Tc, it follows that :-
where, Ta = the DB temperature of the air entering the coil (oC)
Tb = the DB temperature of the air leaving the coil (oC)
Tc = the mean effective temperature of the coil surface (oC)
The BPF (1) increases as the velocity of air over the coil increases; (2) decreases as the pitch of the fin decreases and (3) decreases as the number of rows of the coil increases.
The mixing of air streams can take place when :-
- The fresh air drawn into air handling unit is mixed with re-circulated room air prior to conditioning and supply into the building.
- Supply air is blown through supply grilles or diffusers into the room.
- Exhaust air is discharged from the building into the external atmosphere.
- The discharge air from an external dry or evaporative cooling tower is released into the atmosphere.
- Steam from cooking equipment is released.
- Opening doors between rooms connects areas held at different temperature, humidity or pressure states.
- Air is extracted from different rooms and is mixed in the recirculation ductwork.
- Air from different parts of the same room is drawn towards the extract grill and into ductwork.
Whenever such a mixing process take place, changes occur in the air condition. In comfort air conditioning, room air extracted into the return-air duct may or may not accurately represent the room-air condition around the occupants owing to the relative mixing of air from different parts of the room. It is important to summate mass flow rates for accurate results although an approximation can be made by adding volume flow rates. The inaccuracy here will be due to the changes in air density as the temperature change during the mixing process. The mixed air condition lies on a straight line connecting the two end states. Where,
Q1 = volume flow rates of fresh air (m3/s)
Q2 = volume flow rates of return-air (m3/s)
Q3 = volume flow rates of mixed air (m3/s)
An approximation to the mixed air temperature can be made from the volume flow rates and DB temperatures of the two streams, often with sufficient accuracy for plotting on the psychrometric chart, that is to within 0.5oC DB, by making the assumption that the air densities and specific heat capacities remain practically constant. Divide each side by Q3;
t1 = DB temperature of fresh air (m3/s)
t2 = DB temperature of return-air (m3/s)
t3 = DB temperature of mixed air (m3/s)